Pane of transparent material having a low emissivity

ABSTRACT

A system of layers applied to a transparent substrate includes a first layer of an oxide such as ZnO or SnO 2 , a second layer of a substoichiometric oxide of Zn or Ta, a third layer of Ag or Cu, a fourth layer of a substoichiometric oxide of Ti, Cr, or Nb, and a fifth layer of similar composition as the first layer. The layers are preferably deposited by magnetron cathode sputtering in an atmosphere which consists of inert gas and, in the case of the oxide layers, a reactive gas.

BACKGROUND OF THE INVENTION

The invention pertains to a pane of transparent material with hightransparency in the visible range and with very high reflectivity in thethermal radiation range and also to a process for its production.

Panes of this type should have high chemical resistance to moisture,especially to NaCl-water and SO₂ -water solutions in certainconcentrations.

The invention also pertains to the production of a pane of this type bya coating process based on cathode sputtering.

Windows with panes of this type should in particular prevent radiantheat from escaping from a room to the outside in winter. Known layersystems of this type are referred to as "low-e" ("low emissivity").

Conventional low-e systems consist of various categories of layers,which are designed to have different properties and which are expectedto perform different functions in the system:

(a) a layer with high conductivity for electricity, often consisting ofa metal such as Ag, Au, or Cu, but with a very low radiation emissioncoefficient, represents the actual low-e (low-emissivity) coating;

(b) but because a metal layer is also highly reflective to light (a lowdegree of light transmission) in the visible range, additionaltransparent layers are deposited to reduce its reflectivity. Otherfunctions of these transparent layers are to provide the desired colortone and to give the system a high level of mechanical and chemicalresistance;

(c) to protect the thin metal layer against aggressive atmospheres inthe environment both during and after the production process and also toensure the good adhesion of the adjacent oxide layer, a so-calledblocker layer (barrier layer, primer layer) of a metal or suboxide isoften applied to this metal layer (Ag, Au, Cu).

To accomplish all these tasks, a conventional low-e coating is built upof the following components:

    ______________________________________    substrate | oxide | Ag | blocker |    oxide    ______________________________________

where the substrate is a pane of transparent inorganic or organic glassor a transparent organic film; Ag is an electrically conductive layer;the oxides form the antireflective coating; and the blocker forms aprotective layer for the Ag and also serves as a bonding agent withrespect to the oxide layer.

The light transmission of a conventional low-e coating on a 4-mm glasssubstrate is approximately 80-86%. The thermal transmission through apane of glass such as this depends on the emissivity ( of the low-ecoating and can be described here by means of the simple formula:

    ε≅0.0141×R.sub.▪,

where R.sub.▪ =ρ/d, and

R.sub.▪ =the surface resistance of the silver layer;

d=the thickness of the layer; and

ρ=the resistivity.

The above formula describes the emissivity of a thin metal layer withsufficient accuracy as long as the value is smaller than 0.2. For theknown low-e coatings, ε is approximately 0.1.

The lower the emissivity, the smaller the radiation losses through thecoating. The emissivity can be suppressed either by lowering theresistivity or by increasing the thickness of the layer. When thethickness of the layer is increased, the amount of light which isabsorbed also increases, which leads to an undesirable reduction in theamount of light transmitted. A reduction in the resistivity of the Aglayer, however, leads not only to a reduction in the emissivity but alsoto an increase in the amount of light transmitted.

The resistivity of a thin layer can be described as follows:

    ρ=ρ.sub.K +ρ.sub.F +ρ.sub.G,

where:

ρ_(K) is the resistivity of a monocrystalline layer of infinitethickness;

ρ_(F) is the component of the resistivity caused by electron scatteringalong the layer surfaces; and

ρ_(G) is the component of the resistivity caused by electron scatteringalong the grain boundaries of the individual crystalline grains.

The resistivity ρ_(K) of the very thick, monocrystalline Ag layerdepends on the purity of the metal. Even a very small amount of foreignmaterial can considerably increase the resistance of the layer. Thismeans that the sputtering process should be carried out in a gasatmosphere of such a kind that none of its atoms is introduced into thesilver layer.

The resistivity ρ_(F) of a thin layer depends on the roughness of thelayer surfaces. It is important for the lower oxide layer, on which thesilver grows, to be very smooth. Thus this component of the electronscattering can be significantly reduced.

The resistivity ρ_(G) depends on the size of the grains and on the typeof grain boundaries between the individual grains. The smaller thegrains and the wider and denser the grain boundaries, the greater theelectron scattering. The size of the silver grains can be influenced bysuitable preparation of the substrate surface. The oxide under thesilver should promote the growth of the silver, which will lead tolarger grains. In addition, the oxide elements may not diffuse into thesilver layer. Foreign atoms diffuse into a layer primarily along thegrain boundaries, which leads to an increase in density and thus togreater electron scattering.

SUMMARY OF THE INVENTION

The present invention increases the conductivity of the silver layer ina low-e coating and thus achieves a pane of insulating glass with betterthermal insulating properties. This is done without any reduction in thelight transmission and without any impairment to the mechanical orchemical resistance of the overall coating.

According to the invention, an additional thin layer is provided underthe silver layer, which ensures a very smooth surface, the atoms of thisadditional layer diffusing to only a very slight extent if at all intothe silver. The material of the additional layer is also selected sothat it promotes the growth of the silver. In this way, the conductivityof the Ag layer can be increased by up to 30%. Suitable layer materialsinclude substoichiometric oxides of the metals Zn and Ta and mixturesthereof.

A layer system according to the invention is built up as follows:

    ______________________________________    substrate | oxide | TaO.sub.x | Ag |    blocker | oxide  (1)    substrate | oxide | ZnO.sub.x | Ag |    blocker | oxide  (2)    substrate | oxide | ZnTaO.sub.x | Ag    | blocker | oxide                              (3)    substrate | oxide | ZnO.sub.x | Ag |    blocker |        (4)    oxide | ZnO.sub.x | Ag | blocker |    oxide    ______________________________________

The individual thicknesses in Examples 1-3 are:

first oxide layer, about 40 nm;

second layer, about 4 nm;

the Ag layer, about 6 nm;

the blocker layer, about 1.5 nm; and

the last oxide layer, about 38 nm.

System (4) comprises two Ag layers. As a result of the second Ag layer,the electrical conductivity of the layer package is approximatelydoubled.

The surprising discovery has been made that a thin, substoichiometricZnO_(x), TaO_(x), or ZnTaO_(x) layer can significantly increase theconductivity of Ag and in addition acts as a highly effective bondingagent between Ag and the oxide layer. The mechanical and chemicalresistance of the system is ensured by the blocker on the silver layer.

Panes according to the invention can be produced in an especiallyadvantageous manner in that the layers are applied under vacuum by meansof magnetron cathode sputtering. When a continuous system is used, theprocess makes it possible to coat large panes of glass veryeconomically. The metal layers are applied by sputtering in anoxygen-free atmosphere. The oxide layers and the substoichiometric Zn orTa oxides or the oxides of their alloys can be applied by means ofreactive magnetron cathode sputtering of metallic or alloy targets in anoxygen-containing atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of resistance versus thickness of a ZnO_(x) coating;

FIG. 2 is a plot of surface resistance versus thickness of the Ag layer,with and without ZnO_(x) ; and

FIG. 3 is a plot of transmission plus reflection of the low-e coatingversus thickness of the Ag layer, with and without ZnO_(z).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a sputtering system according to Example I (Table I), the followinglayers were applied in succession to a 2-mm-thick pane of float glassmeasuring 50×50 mm:

a BiAlO_(x) layer in a thickness of 22 nm by the reactive sputtering ofa BiAl target with 4 atom % of Al in an argon-oxygen atmosphere at apressure of 3×10⁻³ mbar;

an Ag layer with a thickness of 12 nm by the sputtering of an Ag targetin an argon atmosphere at a pressure of 3×10⁻³ mbar;

a substoichiometric TiO_(x) layer with a thickness of 2 nm by thesputtering a Ti target in an argon-oxygen atmosphere at a pressure of3×10⁻³ mbar; and

a SnMgO₂ layer in a thickness of approximately 38 nm by the reactivesputtering of a SnMg target with 10 atom % of Mg in an argon-oxygenatmosphere at a pressure of 3×10⁻³ mbar.

In Examples 2-6 (Table I), only the thickness of the second layer(ZnO_(x)) was changed.

The exact values of the individual layer thicknesses and the measuredlight transmission, reflection, and surface resistance values of thelayer systems thus produced are listed in Table I. FIG. 1 shows themeasured surface resistance of the layer systems produced as a functionof the thickness of the ZnO_(x). It can be seen that, as the thicknessof the ZnO_(x) increases up to about 4 nm, the resistance of the Aglayer decreases and then remains constant. The measured increase in theconductivity of. the Ag is greater than 30%.

Table II shows three pairs of low-e coatings. The samples of one pairdiffer only in that one of them includes a ZnO_(x) layer while the otherdoes not. It can be seen that the thin ZnO_(x) layer not only increasesthe electrical conductivity but also improves the optical properties.This is especially evident upon consideration of the sum of T_(y)+R_(y). This value is always higher for the sample which includes theZnO_(x) layer. The difference increases with increasing conductivity ofthe Ag layer. The transmission and reflection values cannot be easilycompared with each other, because the individual layer systems were notoptimized in terms of their optical properties. In this case, only thesum T_(y) +R_(y) is relevant to the evaluation.

                                      TABLE I    __________________________________________________________________________       BiAl-Oxide             ZnO.sub.x                Ag  TiO.sub.x                       SnMg-Oxide                             R  Ty Ry Ty + Ry    No.       (nm)  (nm)                (nm)                    (nm)                       (nm)  (Ω)                                (%)                                   (%)                                      (%)    __________________________________________________________________________    1  22    -- 12  2  38    6.0                                82.2                                   1.1                                      89.3    2  22    1  12  2  38    5.0                                82.8                                   6.3                                      89.1    3  22    2  12  2  38    4.5                                83.8                                   6.2                                      90.0    4  22    3  12  2  38    4.0                                83.8                                   6.2                                      90.0    5  22    4  12  2  38    3.9                                84.8                                   6.5                                      91.3    6  22    5  12  2  38    3.9                                85.3                                   5.6                                      91.0    7  22    6  12  2  38    3.8                                85.0                                   5.6                                      91.0    __________________________________________________________________________

                                      TABLE II    __________________________________________________________________________       BiAl-Oxide             ZnO.sub.x                Ag  TiO.sub.x                       SnMg-Oxide                             R  Ty Ry Ty + Ry    No.       (nm)  (nm)                (nm)                    (nm)                       (nm)  (Ω)                                (%)                                   (%)                                      (%)    __________________________________________________________________________     1 22    -- 12  2  38    6.0                                82.2                                   1.1                                      89.3     7 22    6  12  2  38    3.8                                85.0                                   5.6                                      91.0     8 22    -- 13  2  38    5.4                                82.7                                   5.4                                      88.1     9 22    6  13  2  38    3.5                                83.5                                   6.1                                      89.7    10 22    -- 14  2  38    4.5                                82.3                                   4.3                                      86.6    11 22    6  14  2  38    3.2                                83.0                                   6.5                                      89.5    __________________________________________________________________________

Table III shows low-e systems in which SnMg oxide was used for the twoantireflective layers. In this case, too, the layer with ZnO_(x) hasmuch greater conductivity and a larger total for T_(y) +R_(y).

                                      TABLE III    __________________________________________________________________________       BiAl-Oxide             ZnO.sub.x                Ag  TiO.sub.x                       SnMg-Oxide                             R  Ty Ry Ty + Ry    No.       (nm)  (nm)                (nm)                    (nm)                       (nm)  (Ω)                                (%)                                   (%)                                      (%)    __________________________________________________________________________    12 40    -- 90  2  42    8.0                                83.6                                   3.7                                      87.3    14 40    6  90  2  42    6.4                                84.5                                   4.5                                      89.0    __________________________________________________________________________

What is claimed is:
 1. Pane of transparent material with a substrate anda layer system provided on one side of the substrate, wherein said layersystem comprises(a) a first layer selected from at least one of ZnO,SnO₂, In₂ O₃, Bi₂ O₃, TiO₂, ZrO₂, Ta₂ O₅, SiO₂, Al₂ O₃, or selected fromone at least one of AlN or Si₃ N, or selected from at least one of theoxynitrides of aluminum, titanium, zirconium, and silicon, is depositedin a thickness of 20-70 nm on the substrate; (b) a second layercomprises at least one oxide selected from the group consisting of Ta₂O_(x) and a mixture comprising ZnO_(x) and TaO_(x) in a thickness of 1to 9 nm in the first layer wherein X is a number that results in asubstoichiometric ratio of oxygen to either Zn or Ta; (c) a third layerselected from at least one of Ag and Cu is applied in a thickness of5-30 nm on the second layer; (d) a fourth layer of at least one of themetals Ti, Cr, and Nb, or an alloy with at least 15 atom % of one of Ti,Cr, and Nb is deposited on said third layer as one of a metal layer or asubstoichiometric metal oxide layer in a thickness of 0.5-5 nm, and (e)a fifth layer selected from at least one of ZnO, SnO₂, In₃ O₃, Bi₂ O₃,TiO₂, ZrO₂, Ta₂ O₅, SiO₂, Al₂ O₃, or selected from one at least one ofAlN or Si₃ N, or selected from at least one of the oxynitrides ofaluminum, titanium, zirconium, and silicon is deposited in a thicknessof 20-70 nm on the substrate is deposited on said fourth layer.
 2. Paneaccording to claim 1, wherein at least one of the first and the fifthlayers consists of at least one of SnO₂, In₂ O₃, TiO₂, and Bi₂ O₃, and0-20 atom % of at least one oxide of Mg, Al, P, Ti, Y, Zr, and Ta. 3.Pane according to claim 1 wherein at least one of the first and fifthlayers consists of at least one of SnO₂, In₂ O₃, TiO₂, and Bi₂ O₃, and0-5 atom % of an oxide of an element having an atomic number of 57-71.4. Pane according to claim 1, wherein the second layer has a thicknessof approximately 5 nm.
 5. Pane according to claim 1, wherein anadditional layer is deposited between the third and the fourth layer,said additional layer being a substoichiometric oxide of at least one ofZn and Ta in a thickness of 1-40 nm.
 6. Pane according to claim 1,wherein said substrate is a transparent inorganic glass.
 7. Paneaccording to claim 1, wherein said substrate is a transparent organicglass plate.
 8. Pane according to claim 1, wherein said substrate is atransparent organic film.
 9. Pane according to claim 1, wherein at leastone of the first and the fifth layers is deposited as a double layer oftwo different materials with a total thickness of 20-70 nm.
 10. Paneaccording to claim 1, wherein an additional layer having a thickness of0.5 to 5 nm is deposited between the first and second layers, saidadditional layer being a substoichiometric oxide of at least one of Ti,Cr, and Nb, or of an alloy with at least 15 atom % of one of Ti, Cr, andNb.